Transcranial magnetic stimulation (“TMS”) uses an induction coil in which a time-varying magnetic field is generated to induce an electric field (“E-field”) within the brain. Neurons at the locations of the brain exposed to a strong enough E-field will become activated, or stimulated. In navigated brain stimulation (“NBS”), the E-field induced in the brain by a TMS induction coil device is shown as an overlay on a graphical display of an anatomical representation of the subject's brain. By viewing the display, a user can visualize the E-field induced on the brain and, therefore, interactively position the TMS coil device, in real time, in relation to the brain to stimulate a target site on the brain.
The following data acquisition and processing steps are typically performed as part of NBS.
1. A segmented data representation of the scalp or head surface of a subject is generated from data representative of the anatomical configuration of the subject's head. Typically, data representative of two-dimensional (“2D”) magnetic resonance imaging (“MRI”) images of the head of the subject, which was previously obtained using conventional MRI techniques, and where the images include at least the brain, upper parts of the skull and attached tissue and cartilage, are processed, using well known software algorithms, to generate a volumetric, three-dimensional (“3D”) representation of the head. The 3D representation of the head is then further processed, also using well known software algorithms, to generate a segmented data representation of the head surface of the subject.
2. Tracking elements are implemented to provide that the location and orientation of a TMS coil device with respect to a subject's head can be tracked. As conventional in the art, easily identifiable, reflective markers (trackers) are placed on selected points on the subject's head and also on the TMS coil device to permit automatic recording of the coordinates of the points in 3D. When the coordinates of at least three such points on the head and at least three such points on the TMS coil device have been recorded, the coordinate values for all six degrees of freedom of these objects have been determined. For example, the trackers on the TMS coil device may be a part of a tracking device attached to the TMS coil device, as described in U.S. patent application for TRANSCRANIAL MAGNETIC STIMULATION INDUCTION COIL DEVICE WITH ATTACHMENT PORTION FOR RECEIVING TRACKING DEVICE, Ser. No. 11/847,511 filed Aug. 30, 2007, assigned to the assignee of this application and incorporated by reference herein. The coordinates of the trackers are recorded using a special-purpose camera, as conventional in the art.
3. A co-registration procedure is performed, which correlates data representative of the positions of the trackers on the TMS coil device and the subject's head during a tracking calibration with the image data from which the 3D representation of the subject's head is generated. Typically, in a tracking calibration, several landmark points on the head at which reflective trackers are positioned, such as points on each ear and the nose, are pinpointed on the 2D MRI images or, if available, the volumetric 3D image of the head. The same points are also pinpointed on the subject's head by use of a digitization pen tracker, which also includes reflective trackers. Tracking data representative of the positions of the trackers on the TMS coil device and the subject's head when each of the points are pinpointed by the pen tracker are then collected. After performing such point-to-point correspondences or point-to-point matching, a transformation is computed that aligns the coordinate system used to represent the head in the MRI image data with the coordinate system used to represent the relative positions of the trackers during the calibration. The quality of the transformation can be enhanced, for example, at least in the least-squares sense, by performing additional point-to-point matching which, in turn, improves the accuracy of NBS.
4. On an NBS display, the following are typically shown: a graphical representation of the TMS coil device, in particular preferably only the casing of the TMS coil device in which the coil windings are contained, in relation to a graphical representation of the scalp; a graphical representation of a portion of the brain at a selected depth; and the E-field induced on the brain portion by the TMS coil device as an overlay on the representation of the brain portion. The display, thus, provides the user with a visual representation of the position and orientation of the casing, and thus the coil windings of the TMS coil device, in relation to the head and the brain, and also the E-field induced in the brain, as the user navigates the TMS coil device in relation to the subject's head. The quality of the transformation computed in the co-registration (3. above) affects the accuracy of the representations shown on the display and, thus, the navigation accuracy. As is well known in the art, the E-field induced by the coil windings is computed using a head-shape or head-conductivity-distribution model, e.g., a spherical model, such as described in Ravazzani, P., et al., “Magnetic stimulation of the nervous system: induced electric field in unbounded, semi-infinite, spherical, and cylindrical media,” Annals of Biomedical Engineering 24: 606-616, 1996, incorporated by reference herein, and based on a model of the shape and location of the copper windings inside the casing of the TMS coil device. The E-field is typically shown on the representation of the brain portion using colors to indicate E-field strength, which aid the user in navigating the TMS coil device to stimulate target sites on the brain portion. The accuracy of the representation of the brain portion, in a large part, determines the accuracy of the representation of the E-field induced on the brain portion shown on the display and, thus, greatly impacts the accuracy with which the user can navigate the TMS coil device to stimulate target sites on the brain.
Prior art co-registration techniques, however, are prone to errors, which can cause inaccuracies in the display of the position of the TMS coil device and the resulting E-field on the NBS display and, thus, cause inaccuracies during NBS. The most critical sources of error in co-registration are the following.
1. Inaccurate matching of the physical head shape and the MRI images. MRI image data collection typically includes a geometrical distortion that affects the detected head shape. For example, the geometric distortion may cause a perfect sphere to appear as an ellipsoidal on a display. In addition, data segmentation of the MRI image data may not precisely identify the scalp, because of inaccuracies in the MRI image data in the gray level values of the voxels representing points near the scalp surface. As currently obtainable MRI image data typically only has enough resolution to provide for the generation of a 3D representation of the head volume using 1 mm×1 mm×1 mm voxels, the obtainable voxel size limits the accuracy of the MRI image data representative of only the head. In addition, the MRI data includes other inaccuracies that are sources of the geometrical distortion.
2. Trackers on the head move with respect to the head during the NBS procedure. If the trackers on the head move with respect to the head, the co-registration is invalidated and must be repeated. Movement of the trackers on the head may not be detected when a patient is undergoing a NBS procedure using a TMS coil device. For ordinary NBS patient procedures, it is not feasible or practical to fix the trackers to the head of the patient, which may require screwing the trackers into the head.
3. Point-to-point matching is not possible. As the MRI image data has limited resolution, for example, providing for a volumetric representation of the head where the voxels are 1×1×1 mm3, it is very difficult, if not impossible, to pinpoint exactly the same points on the head and also on the volumetric head representation generated from the MRI image data. The inaccuracy in the selection of landmark points during a tracking calibration leads to navigation errors during NBS.
Inaccurate matching of the MRI image data with the subject's head, and inaccurate navigation of the TMS coil device in relation to the head, are readily visible on an NBS display. For example, during NBS, a user of the TMS coil device normally places the TMS coil device so that its casing touches the scalp during stimulation. When the user places the TMS coil device on the scalp, with the outer surface of the casing of the TMS coil device touching the scalp, the TMS coil device should appear to touch the scalp, where the coordinate system used to represent the positions of the trackers on the TMS coil device and the head is accurately co-registered with the coordinate system for the MRI image data representative of the head. The above-mentioned errors in co-registration, however, typically cause the TMS coil device to appear on the NBS display either above the scalp or inside the head, the latter of which is impossible.
It is undesirable to stop an NBS procedure, and repeat the collection of tracking information from the trackers as part of another tracking calibration, such that another transformation can be performed and, therefore, provide that the NBS display shows the TMS coil device in relation to the head based on an updated co-registration.
Therefore, there exists need for correcting an error in the co-registration of a coordinate system used to represent the head in the MRI image data with the coordinate system used to represent the position of trackers on the head and a TMS coil device, without collecting additional tracking data following a tracking calibration.